1. Field of the Invention
The invention is directed to compositions of display cell structure and electrode protecting layers for improving the performance of display devices.
2. Description of Related Art
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by spacers. One of the electrodes is usually transparent. An electrophoretic fluid composed of a colored solvent with charged pigment particles dispersed therein is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side or the other causing either the color of the pigment particles or the color of the solvent being seen from the viewing side.
An improved EPD technology was disclosed in U.S. Pat. No. 6,930,818 (corresponding to WO 01/67170), U.S. Pat. No. 6,672,921 (corresponding to WO 02/01281) and U.S. Pat. No. 6,933,098 (corresponding to WO02/65215), all of which are incorporated herein by reference in their entirety. The improved EPD cells may be prepared by a lithographic process or by microembossing a layer of a radiation curable composition coated on a first substrate layer to form microcups of well-defined shape, size and aspect ratio. The microcups are then filled with an electrophoretic fluid and sealed with a sealing layer. A second substrate layer is laminated over the filled and sealed microcups, preferably with an adhesive layer.
The microcapsule type EPD as described in U.S. Pat. Nos. 5,961,804 and 5,930,026 has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic fluid of a dielectric solvent and a suspension of charged pigment particles that visually contrast with the dielectric solvent. The microcapsules may be immobilized within a transparent matrix or binder that is itself sandwiched between two electrodes.
For all types of electrophoretic displays, image bistability is one of the most important issues. However in certain cases, the image bistability may degrade due to reverse bias. The term “reverse bias” is commonly used to describe a voltage induced by the capacitor discharge effect from a dielectric material used in an electrophoretic display. The polarity of the reverse bias is opposite of that of the applied driving voltage and therefore the reverse bias may cause the particles to move in a direction opposite from the intended direction. As a result, the display may have inferior image contrast and bistability. An example of reverse bias is illustrated in FIG. 1. The voltage sensed by the electrophoretic fluid when the applied voltage drops from +40V to 0V is referred to as the “reverse bias” and its polarity is negative (opposite of the applied voltage).
The dielectric materials referred to above are usually used for the formation of the display cell structure and thin polymer layer(s) between the electrode plates and the electrophoretic fluid. For the microcapsule type displays, the polymer layer may be an adhesive layer, the microcapsule wall, the polymer matrix in which the microcapsule-based display cells are dispersed, or a tie layer. For the microcup type displays, the polymer layer may be an adhesive layer, the sealing layer, a layer between the microcups and the bottom electrode plate (i.e., the primer layer), or a tie layer. The term “polymer layer” referred to herein may also be referred to as an “electrode protecting layer” or a “dielectric layer”, in the context of the present application.
In order to achieve desired electrical properties of an electrophoretic display, the resistivities of the display cell structure and the polymer layer(s) of the display must be controlled. The basic principle of achieving the desired electrical properties of an electrophoretic display involves increasing the resistivity of the electrophoretic fluid and/or lowering the resistivities of the display cell structure and/or the polymer layer(s). While the room for increasing the resistivity of the electrophoretic fluid is limited, lowering the resistivities of the display cell structure and/or the polymer layer(s) appears to be a more promising option.
U.S. Pat. No. 6,657,772 discloses that the volume resistivity of an adhesive layer can be decreased by blending a conductive filler into an adhesive composition. However, it also acknowledges that there are great difficulties in adopting this approach to achieve the volume resistivity of about 1010 ohm cm required for an adhesive layer used in an electrophoretic display. The reference further states that the volume resistivity of the conductive filler should not be about two orders of magnitude less than the intended volume resistivity of the final blend and it claims that an adhesive layer that has a volume resistivity in the range of about 109 to about 1011 ohm cm may only be achieved by a mixture of an adhesive material having a volume resistivity of at least about 5×1011 ohm cm and a filler having a volume resistivity not less than about 107 ohm cm.
Most conductive fillers which may be incorporated into a dielectric material are not transparent and require tedious grinding or milling to be uniformly dispersed into the composition for the display cell structure and polymer layer(s). Aggregation of the filler particles may also results in an undesirable effect such as poor image uniformity, mottling or, sometimes, short circuit of the display.
In the case of microcup-based electrophoretic displays, incorporation of conductive filler particles into the composition of the microcup structure or the electrode protecting layer(s) tends to cause problems in the manufacture of the microcups. Defective microcups may be resulted from insufficient or non-uniform degree of photoexposure during the microcup forming process (e.g., microembossing or photolithographic exposure). Moreover, if the particle size of the filler particles is relatively large as compared to the degree of surface roughness or thickness of the layer comprising the particles, damage on the embossing shim or the conductor film (such as ITO/PET) during embossing may be observed, particularly when the hardness of the conductive filler is higher than that of the shim material or conductor film used.
Low resistance fillers (such as metal oxides and polyether block amide elastomers), in the past, have been added into a polymer composition to form an interconnecting or percolation network in order to reduce the volume resistivity of the polymer structure formed from such a polymer composition. However, the interconnecting or percolation network in this approach is always a two-phase system, with the low resistance filler as the dispersed phase. Optically, the two-phase system often causes a change of the appearance of the polymer structure, such as increased opacity. If the low resistance filler is not well dispersed, the polymer structure formed will show a dark color. Furthermore, a system based on a percolation network often requires the loading of the low resistance filler to be over, but still close to, the percolation threshold, at which stage the filler particles are close enough to touch each other but still not enough to dominate the resistance of the system. As a result, the resistance of the system undergoes a sharp transition from the resistance of the continuous phase to that of the dispersed phase. In practice, formulating such a system to achieve an exactly desired level of electrical property with consistency can be difficult because the success of the system would depend on a variety of factors, such as the aggregation structure and exact loading of the filler particles and the amount of impurity that comes with the filler material.